In a simple type of conventional fluid mixing apparatus, a fluid is fed by a line or tube to a stirred vessel containing another liquid. Mixing equipment of this kind has been used for liquid phase chemical reactions and for physical mixing of liquids as in the formation of colloidal suspensions. Examples of mixing equipment are found in U.S. Pat. Nos. 4,289,733, 3,692,283, 3,415,650 and in Japanese Patent No. 58289 and Japanese Patent Application No. 275023. In addition to having mixing means that uniformly mix the fluids, it is desirable to feed the fluids to the mixers in a uniform manner. This is the subject of the present invention.
The making of silver halide photographic emulsions is an example of an operation that requires highly efficient distribution and mixing of liquids. As described in Chapter 3 of "The Theory of the Photographic Process," 4th Edition, T.H. James, Editor, silver halide crystals or grains are precipitated and dispersed in a colloid or peptizer solution, which normally is gelatin. The silver halide is formed by the reaction of a solution of a halide salt, e.g., potassium bromide, with a solution of a silver salt, usually silver nitrate. Two common methods of mixing these components are the single-jet and double-jet methods.
In the double-jet method, aqueous solutions of the silver salt and the halide are added simultaneously by separate feed lines to a stirred vessel which contains the aqueous gelatin solution. With conventional apparatus the concentrations of reactants are not uniform throughout the process and the silver halide grain sizes and shapes vary considerably. For silver halide emulsions of the highest quality a narrow range of grain sizes and shapes is necessary. Even a small concentration of large grains in a fine grain emulsion can cause such problems as reduced photographic contrast or a defect known as "pepper fog." Similar problems occur in the single-jet technique using conventional apparatus wherein a silver nitrate stream is added to a gelatin solution which contains the alkali metal halide.
One way to improve the mixing of liquids is to feed the stream or streams to the mixing zone by means of a distributor having multiple orifices instead of by a single line or tube. See, for example, FIG. 4 of the patent to Brogli et al., U.S. Pat. No. 3,925,243. Although intended to improve distribution of the liquid stream, such a single distributor is not useful over a broad range of flow rates. For variations in feed rates, the diameter of the feed line and the cross-sectional area of the distributor channels and orifices must be large enough to provide an acceptable pressure drop at the highest flow rate to be encountered. Consequently, the velocity in the feed line and distributor orifices will be unacceptably low at the lowest flow rate. This can lead to axial mixing within the feed line, distributor channels and orifices themselves as a result of laminar flow or density inversions (if the reactant has a density different from that of the fluid initially in the feed line). Also a long time may be required to fill the feed line and distributor at low flow rates. This can vary the time at which reactants arrive at the distributor orifices. When multiple reactants are being delivered and their simultaneous arrival at the start of the reaction is critical to the reaction, axial mixing can also decrease the quality and yield of the desired reaction product.
Another common drawback of conventional distribution apparatus is that the liquid or fluid feed is not uniformly distributed in the vessel. Consequently, for chemical reactions that require uniform distribution to form the desired reaction product, the product yield or quality is poor. If the feed rate decreases and the pressure loss through the orifices in a conventional distributor is less than or approximately equal to variations in the pressure field created by the agitator of the mixing vessel, uneven distribution of reactant flow can occur, and in the worst case back flow occurs into the distributor, with detrimental effect on the reaction product.